Extreme-ultraviolet lithography (EUVL) is currently regarded as a candidate “next generation lithography” (NGL) that offers prospects of substantially finer pattern resolution than currently obtainable using conventional “optical” lithography (i.e., lithography performed using deep-ultraviolet wavelengths of light). These expectations of increased resolution from EUVL stem largely from the fact that, whereas current optical lithography is performed using a wavelength in the range of 150-250 nm, EUVL is performed using a wavelength in the range of 11-15 nm, which is at least ten times shorter than the conventional “optical” wavelengths. Generally, the shorter the wavelength of light used for pattern imaging in microlithography, the finer the obtainable resolution.
In view of the extremely small pattern elements (currently less than 100 nm) that can be resolved using microlithography, including EUVL, the accuracy and precision with which pattern transfer is performed lithographically must be extremely high to ensure proper placement and registration of multiple pattern layers on a substrate and to ensure that the pattern elements are transferred to the substrate with high fidelity. To obtain such high accuracy and precision, extreme measures are taken to control and remove extraneous causes of performance degradation. For example, with current expectations being demanded of microlithography systems to produce pattern features of less than 100 nm, eliminating significant particulate contamination has become paramount.
EUV light is highly attenuated by the atmosphere, and no currently known materials are adequately transmissive and refractive to EUV light for use as EUV lenses. Consequently, EUVL must be performed under partial vacuum conditions using reflective optics (mirrors) for illumination of the reticle and for projection of the illuminated pattern from the reticle to the substrate. Even the reticle is reflective rather than being a transmissive reticle as used in conventional optical microlithography.
In optical microlithography the reticle during use typically is protected by a pellicle from particulate contamination. (The pellicle is a transmissive thin film on a frame that covers the patterned surface of the reticle to prevent deposition of particles on the reticle surface.) A pellicle cannot be used with a reticle for EUVL because, in view of the lack of EUV-transmissive materials, the pellicle would absorb and thus block the EUV beam incident to the reticle, leaving substantially no EUV light for projecting the pattern image to the substrate. Thus, the EUVL reticle must be used naked, which leaves the reticle vulnerable to particulate contamination during use. In optical lithography in which the reticle is protected by a pellicle, a particle deposited on the pellicle is sufficiently displaced from the plane of the reticle (i.e., outside the depth of focus) to be unresolved (or at most poorly resolved) on the wafer. A particle on a naked EUVL reticle, on the other hand, is in the plane of the reticle pattern and hence is resolved on the wafer where it likely will flaw the projected pattern image. Hence, for EUVL reticles, particle protection as effective as a pellicle is required.
In EUVL systems currently under development, the reticle is used facing downward, which is helpful in preventing deposition of particles on the reticle surface. However, merely facing the reticle downward is insufficient for keeping the reticle completely clean, and various techniques currently are being developed that are aimed at preventing deposition of any particulate contamination on the reticle without having to use a pellicle. One technique that exhibited remarkable success in preventing particulate deposition on the reticle (by preventing particles from hitting the reticle) is termed “thermophoresis,” discussed in Rader et al, “Verification Studies of Thermophoretic Protection for EUV Masks,” Proceedings SPIE 4688:182-193, 2002. See also U.S. Pat. Nos. 6,153,044 and 6,253,464. Thermophoresis refers to a force exerted on particles suspended in a gas where a temperature gradient is present, wherein the particles are driven by a thermophoretic “force” (imparted by the gas) from a warmer region to a cooler region. Thus, a surface can be protected from particle deposition by maintaining the surface at a warmer temperature than its surroundings.
General principles of thermophoresis as applied in an EUVL system are described with reference to FIG. 9, which depicts a reticle 222 and a nearby surface 226 that is maintained at a cooler temperature than the reticle 222. The cooler surface 226 may be, for example, a shield that protects the reticle or a differential pumping barrier used in a vacuum chamber housing the reticle 222. A gas in the vicinity of the reticle 222 and the surface 226 exhibits a temperature gradient in which the gas is warmer near the reticle 222 and cooler near the surface 226. The thermophoretic “force” associated with the gradient urges particles 228 away from the warmer reticle 222 toward the cooler surface 226. Some particles 228 may actually become attached to the surface 226. Thermophoretic forces are greatest in the presence of a sufficient gas pressure in which the mean free path of the gas molecules is a small fraction of the distance from the reticle 222 and the surface 226. As pressure is decreased (i.e., as vacuum is increased), thermophoretic forces decrease correspondingly. In other words, thermophoresis loses effectiveness in high vacuum, but at a pressure of 50 mTorr thermophoresis is still significant for effectively keeping particles 228 away from the reticle 222.
A conventional thermophoretic scheme as disclosed in the references cited above is generally shown in FIG. 10, which depicts a portion of an EUVL system 100 in the vicinity of the reticle. The depicted system 100 comprises a vacuum chamber 104 including a first region 108 and a second region 110. The first region 108 contains a reticle stage 114 that supports a reticle chuck 118 configured to hold a reticle 112 face-down. The second region 110 contains projection optics 124 and a wafer stage (not shown). The first and second regions 108, 110 are substantially separated from each other by a barrier wall 126 through which an opening 130 is defined. The barrier wall 126 and opening 130 collectively form a differential pumping barrier. The opening 130 is sufficiently large to pass EUV light incident to and reflected from the reticle 112. Gas at a pressure of approximately 50 mTorr is supplied to the first region 108 via a gas-supply port 132 in the vacuum chamber 104. To minimize EUV-absorption losses to ambient gas, the second region 110 is maintained at a lower pressure (i.e., higher vacuum; e.g., ≦5 mTorr) than the first region 108. Maintaining these two respective pressures in the regions 108, 110 is achieved by differential evacuation of the regions, performed using respective vacuum pumps 134, 136 and facilitated by the differential pumping barrier.
In the configuration shown in FIG. 10, to urge particles away from the reticle 112 by thermophoresis, the reticle is maintained at a higher temperature than the barrier wall 126. This temperature differential, as discussed above, results in attraction of the particles to the barrier wall 126, which causes some particles (entrained in gas passing through the opening 130) to enter the second region 110 via the opening 130. The flow of gas from the region 108 to the region 110 also helps convey particles away from the reticle 112 and thus prevents the particles from contacting the reticle.
While placing a cooler surface proximal to a warmer reticle helps reduce particulate contamination of the reticle, maintaining surfaces of different temperatures within the EUVL system can be problematic. For example, maintaining surfaces at different temperatures can complicate temperature control of critical subsystems and can generate issues relating to thermal expansion and distortion of critical components. For example, thermal expansion or distortion of the reticle can compromise the performance of the overall EUVL lithography process and hence of the semiconductor-device-fabrication process. Also, flowing gas from the region 108 to the region 110 may sweep particles originating in the region 108 toward the reticle 112, which would increase the risk of contamination despite the generally enhanced protection afforded by thermophoresis to other regions of the reticle.
One manner of solving this problem is described in U.S. patent application Ser. No. 10/898,475, incorporated herein by reference, filed on Jul. 23, 2004, by the current Applicant. Briefly, a space is defined between the reticle and a nearby surface, such as a barrier wall or reticle shield. Gas nozzles are situated in the space. A gas, cooled to below the temperature of the reticle and surface (the reticle and surface normally have substantially the same temperature), is discharged from the nozzles into the space. The discharged gas, flowing substantially parallel to the reticle and surface, establishes local respective temperature gradients adjacent the reticle and surface. The temperature gradients engender respective thermophoretic forces tending to keep particles entrained in the gas and away from the reticle and surface.
A particular configuration of the apparatus 300 described in the '475 application is shown in FIG. 11, which depicts a reticle 302 supported by a reticle chuck 304 mounted face down on a reticle stage 306. The reticle stage 306, reticle chuck 304, and reticle 302 are contained in a reticle chamber 308 that is separated from a projection-optics chamber 310 by a barrier wall 312 (e.g., a differential pumping barrier or reticle shield). The barrier wall 312 defines a “fixed-blind aperture” 314 that is sized and configured to allow illumination EUV light 316 to impinge on the desired region of the reticle 302 and to pass patterned EUV light 318 reflected from the reticle to downstream projection optics (not shown). The fixed-blind aperture 314 also helps establish and maintain the differential pressures in the two chambers 308, 310. The reticle chamber 308 is typically at approximately 50 mTorr (and thus is a “higher-pressure” region), and the projection-optics chamber 310 is typically at less than 1 mTorr (and thus is a “lower-pressure” region). During exposure, to illuminate successive regions of the reticle 302, the reticle stage 306 moves in a scanning manner relative to the fixed-blind aperture 314. Flanking the fixed-blind aperture 314 and extending upward (in the figure) toward the reticle 302 are nozzle manifolds 320a, 320b that define nozzle openings 322a, 322b for discharging the gas. The nozzle openings 322a, 322b are oriented so as to discharge the gas into the space 324, between the reticle 302 and the barrier wall 312, in a direction substantially parallel to the reticle. The flow of gas (note arrows 326) away from the nozzle openings 322a, 322b past the reticle 302 is substantially laminar. The nozzle openings 322a, 322b may be covered by filters (not shown) that can prevent the admission of particles into the space 324 and can also limit the velocity of gas flow.
As noted above, the gas can be cooled before discharging the gas into the space 324 between the reticle 302 and barrier wall 312. Alternatively, the nozzle openings 322a, 322b are sized and configured to establish a substantially higher gas pressure at the nozzle openings than in the space 324. Thus, discharge of the gas is accompanied by adiabatic cooling of the gas. I.e., as the gas is discharged into the space 324, it expands rapidly out of the nozzle openings 322a, 322b and cools significantly in the process. With such a configuration, the discharged gas is colder than the reticle 302 and barrier wall 312 and establishes the desired temperature gradient without having to pre-cool the gas. In addition, the relatively high gas pressure at the nozzle openings 322a, 322b produces a high gas-flow velocity through the space 324. This high-velocity flow establishes a substantial viscous-drag force on particles and tends to convey the particles out of the space 324 and thus away from the reticle 302.
As indicated by the multiple arrows 326, most of the discharged gas (and entrained particles) flows laterally as shown, substantially parallel to the reticle 302, through the space 324 and is exhausted via the vacuum pump (not shown but see item 134 in FIG. 10) that evacuates the reticle chamber 308.
Referring further to FIG. 11, the nozzle manifolds 320a, 320b extend upward (in the figure) and form respective narrow gaps G between the “tops” of the nozzle manifolds and the surface of the reticle 302. These gaps G, each approximately 1 mm or less, allow limited movement of the reticle 302 (in the vertical, or “Z,” direction) as required for focus control and reticle-wafer alignment movements. The narrow gaps G also allow a limited flow of gas (note single arrows 330 compared to multiple arrows 326) from the space 324 through the fixed-blind aperture 314 to the projection-optics chamber 310. The gas flow through the gaps G is limited to maintain the desired vacuum level in the projection-optics chamber 310 for minimal attenuation of the EUV illumination and patterned beams.
Because of the small distance between the reticle 302 and the top of the nozzle manifolds, maintaining a temperature gradient, and hence thermophoretic protection, within the gaps G can be problematic. Therefore, protection of the reticle is somewhat weaker within the gaps G. However, the flow of gas through the gaps G, from the higher-pressure region 308 to the lower-pressure region 310, will provide some viscous drag force to convey particles into the lower-pressure region 310 and away from the reticle 302. Also, during normal reticle scanning, a given area of the reticle 302 spends only a fraction of the time within the gaps G. Much of the time the reticle lies within the space 324 in which thermophoretic protection and gas drag are available.
In a conventional EUVL system, illumination of the reticle 302 is non-telecentric. Consequently, movement or displacement of the reticle 302 in the axial direction (vertical direction in the figure) causes corresponding image movement at the wafer, which is problematic. Consequently, the “height” of the reticle 302 must be controlled very accurately and precisely to avoid image distortion at the wafer. An example specification for reticle-height control is 50 nm peak-to-valley over the surface of the reticle 302. Achievement of such height control requires corresponding measurements of reticle height, which is performed using a very accurate and precise autofocus (AF) system at the reticle 302.
Accurate measurements of reticle height performed using an AF system require that the AF system be calibrated periodically such as during use of the reticle 302 and whenever a new reticle is mounted to the reticle chuck 304. The AF-system calibration involves scanning the patterned regions of the surface of the reticle 302 with an array of multiple light beams (e.g., 50-70 individual laser beams, at near-grazing incidence on the reticle surface). The beams are reflected from the reticle surface, which is accompanied by some diffraction and scattering of the beams. The reflected beams propagate to respective sensors. At each sensor the respective position of the reflected beam is a function of the reticle “height” at the particular incidence locus of the beam on the reticle. The sensor outputs are averaged to obtain data concerning the mean height of the area actually being measured. The calibration covers an area of the reticle 302 that is larger than the area illuminated at any instant by the EUV illumination beam (i.e., larger than the opening of the fixed-blind aperture 314). Consequently, the fixed-blind aperture 314 (with nozzle manifolds 320a, 320b) is moved out the way (retracted) for the AF-system calibration.
For reasons discussed more thoroughly later below, retraction of the nozzle manifolds 320a, 320b and of the fixed-blind aperture 314 disrupts the gas flow 330 used for establishing differential pressures in the chambers 308, 310 and for providing protection of the reticle 302 in the region of the reticle adjacent the gaps G and fixed-blind aperture. (Thermophoretic protection of other portions of the reticle, namely in the space 324, is maintained.) This situation is shown in the gas-flow image in FIG. 4, which shows a gas flow of approximately 50 m/sec in the space 324 but no gas flow in the gap G. As a result, reticle protection from particulate contamination is compromised and the pressure in the projection-optics chamber 310 is undesirably increased.